There are basically two types of digital computer architectures. The first one is called Von Neumann architecture and later Harvard architecture was adopted for designing digital computers.
Von Neumann Architecture:
It is named after themathematicianand earlycomputer scientistJohn Von Neumann.
The computer has single storage system(memory) for storing data as well as program to be executed.
Processor needs two clock cycles to complete an instruction.Pipelining the instructions is not possible with this architecture.
In the first clock cycle the processor gets the instruction from memory and decodes it. In the next clock cycle the required data is taken from memory. For each instruction this cycle repeats and hence needs two cycles to complete an instruction.
This is a relatively older architecture and was replaced by Harvard architecture.
The name is originated from "Harvard Mark I" a relay based old computer.
The computer has two separate memories for storing data and program.
Processor can complete an instruction in one cycle if appropriate pipelining strategies are implemented.
In the first stage of pipeline the instruction to be executed can be taken from program memory.In the second stage of pipeline data is taken from the data memory using the decoded instruction or address.
Most of the modern computing architectures are based on Harvard architecture.But the number of stages in the pipeline varies from system to system.
These are the basic differences between the two architectures.A more comprehensive list can be found here with respect to ARM class of processors.